### Browsing by Author "Sykora, Steffen"

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- ItemDisorder-induced coupling of Weyl nodes in WTe2(2020) Sykora, Steffen; Schoop, Johannes; Graf, Lukas; Shipunov, Grigory; Morozov, Igor V.; Aswartham, Saicharan; Büchner, Bernd; Hess, Christian; Giraud, Romain; Dufouleur, JosephThe finite coupling between Weyl nodes due to residual disorder is investigated by magnetotransport studies in WTe2. The anisotropic scattering of quasiparticles is evidenced from classical and quantum transport measurements. A theoretical approach using the real band structure is developed in order to calculate the dependence of the scattering anisotropy with the correlation length of the disorder. A comparison between theory and experiments reveals a short correlation length in WTe2 (ξ∼5 nm). This result implies a significant coupling between Weyl nodes and other bands. Our study thus shows that a finite intercone scattering rate always exists in weakly disordered type-II Weyl semimetals, such as WTe2, which strongly suppresses topologically nontrivial properties.
- ItemElastoresistivity of Heavily Hole-Doped 122 Iron Pnictide Superconductors(Lausanne : Frontiers Media, 2022) Hong, Xiaochen; Sykora, Steffen; Caglieris, Federico; Behnami, Mahdi; Morozov, Igor; Aswartham, Saicharan; Grinenko, Vadim; Kihou, Kunihiro; Lee, Chul-Ho; Büchner, Bernd; Hess, ChristianNematicity in heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from magnetic instability were declared in RbFe2As2 and its sister compounds. Here, we report a systematic elastoresistance study of a series of isovalent- and electron-doped KFe2As2 crystals. We found divergent elastoresistance on cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven toward a Lifshitz transition. However, we conclude that none of them necessarily indicates an independent nematic critical point. Instead, the increased nematicity can be associated with another electronic criticality. In particular, we propose a mechanism for how elastoresistivity is enhanced at a Lifshitz transition.
- ItemEvidence for a percolative Mott insulator-metal transition in doped Sr2IrO4(College Park, MD : APS, 2021) Sun, Zhixiang; Guevara, Jose M.; Sykora, Steffen; Pärschke, Ekaterina M.; Manna, Kaustuv; Maljuk, Andrey; Wurmehl, Sabine; van den Brink, Jeroen; Büchner, Bernd; Hess, ChristianDespite many efforts to rationalize the strongly correlated electronic ground states in doped Mott insulators, the nature of the doping-induced insulator-to-metal transition is still a subject under intensive investigation. Here, we probe the nanoscale electronic structure of the Mott insulator Sr2IrO4−δ with low-temperature scanning tunneling microscopy and find an enhanced local density of states (LDOS) inside the Mott gap at the location of individual defects which we interpret as defects at apical oxygen sites. A chiral behavior in the topography for those defects has been observed. We also visualize the local enhanced conductance arising from the overlapping of defect states which induces finite LDOS inside of the Mott gap. By combining these findings with the typical spatial extension of isolated defects of about 2 nm, our results indicate that the insulator-to-metal transition in Sr2IrO4−δ could be percolative in nature.
- ItemHeavy fermion properties of the Kondo Lattice model(London : Nature Publishing Group, 2013) Sykora, Steffen; Becker, Klaus W.We study the S = 1/2 Kondo lattice model which is widely used to describe heavy fermion behavior. In conventional treatments of the model the Kondo interaction is decoupled in favour of a hybridization of conduction and localized f electrons. However, such an approximation breaks the local gauge symmetry and implicates that the local f-occupation is no longer conserved. To avoid these problems, we use in this work an alternative approach to the model based on the Projective Renormalization Method (PRM). Thereby, within the conduction electron spectral function we identify the lattice Kondo resonance as an almost flat excitation near the Fermi surface which is composed of conduction electron creation operators combined with localized spin fluctuations. This leads to an alternative description of the Kondo resonance without having to resort to an artificial symmetry breaking.
- ItemTheoretical approach to resonant inelastic X-ray scattering in iron-based superconductors at the energy scale of the superconducting gap(London : Nature Publishing Group, 2016) Marra, Pasquale; van den Brink, Jeroen; Sykora, SteffenWe develop a phenomenological theory to predict the characteristic features of the momentum-dependent scattering amplitude in resonant inelastic x-ray scattering (RIXS) at the energy scale of the superconducting gap in iron-based super-conductors. Taking into account all relevant orbital states as well as their specific content along the Fermi surface we evaluate the charge and spin dynamical structure factors for the compounds LaOFeAs and LiFeAs, based on tight-binding models which are fully consistent with recent angle-resolved photoemission spectroscopy (ARPES) data. We find a characteristic intensity redistribution between charge and spin dynamical structure factors which discriminates between sign-reversing and sign-preserving quasiparticle excitations. Consequently, our results show that RIXS spectra can distinguish between s± and s++ wave gap functions in the singlet pairing case. In addition, we find that an analogous intensity redistribution at small momenta can reveal the presence of a chiral p-wave triplet pairing.
- ItemUnconventional superconductivity and interaction induced Fermi surface reconstruction in the two-dimensional Edwards model(London : Nature Publishing Group, 2016) Cho, Dai-Ning; van den Brink, Jeroen; Fehske, Holger; Becker, Klaus W.; Sykora, SteffenWe study the competition between unconventional superconducting pairing and charge density wave (CDW) formation for the two-dimensional Edwards Hamiltonian at half filling, a very general two-dimensional transport model in which fermionic charge carriers couple to a correlated background medium. Using the projective renormalization method we find that a strong renormalization of the original fermionic band causes a new hole-like Fermi surface to emerge near the center of the Brillouin zone, before it eventually gives rise to the formation of a charge density wave. On the new, disconnected parts of the Fermi surface superconductivity is induced with a sign-changing order parameter. We discuss these findings in the light of recent experiments on iron-based oxypnictide superconductors.